There are several reasons why battery storage is a valuable addition for many PV systems:
More self-consumed solar power
During the day, PV systems often generate more electricity than the building consumes. The surplus solar power is fed into the public grid. The feed-in tariff is usually lower than the savings you would achieve by using that electricity yourself. With a battery storage system, this energy can be stored and used later when the PV system is not producing any solar power.
Greater independence from the utility provider
Because a larger share of your electricity demand can be covered by your own generated power, you need to purchase less electricity from external suppliers. This also makes you less affected by rising electricity prices. With the right system design, a self-sufficiency level of around 60–80% can be achieved.
Contributing to the energy transition
End users can actively take part in the energy transition at home by generating and consuming more renewable electricity. This reduces the need for power from large centralized plants that may still rely on sources such as nuclear power or lignite coal to produce electricity.
Reducing strain on the public power grid
Grid-balancing concepts can also help relieve and stabilize public power grids. During periods of low demand, electricity can be stored in household battery systems and then discharged quickly when demand rises. This helps smooth out fluctuations in the grid and can reduce the risk of power outages.
Backup / emergency power function
Many inverters, when combined with a battery, offer a backup or emergency power function. This means that in the event of a grid outage, selected loads—or even the entire home—can be supplied with electricity from the battery.
Choosing the right storage system depends on factors such as your individual consumption profile and the applications you want to cover. For an average household of four, around 8 kWh of storage capacity is typically needed. We also offer pre-configured complete packages. Most batteries are available in different capacities and can be expanded later.
To operate a battery storage system, you need an inverter that can connect not only to the PV modules but also to a battery. For retrofits, you will need an inverter that is capable of managing a battery. Manufacturers use different terms for this, such as hybrid inverter, ESS (Energy Storage System), or AC-coupled / battery inverter.
To plan a cost-effective battery storage solution, there are several points to consider. The key is to focus on your own electricity consumption and the goals you want to achieve. For most private households, the main aim is to cover evening, night and early-morning demand with PV power from the battery. A practical approach is to record your meter readings during this period over several days and calculate the difference. This provides a solid basis for estimating the required capacity.
BUT: many people also want power supply during an outage of the public grid. To ensure electricity is available during a blackout, a certain amount of energy must remain reserved in the battery. You should therefore consider how long you want to bridge. Many systems can recharge from PV during an outage, but there are also days or periods with little or no sunshine—especially in winter. The battery size should be adjusted accordingly. More on this further below.
You should also take the size of your PV system into account. Some experts recommend that a lithium-ion battery should be able to charge in around 2 hours, as this has proven to be very beneficial for this battery type. That means the battery should be no more than about twice the size of the PV system. A good rule of thumb is a ratio of around 1.1–1.6 between PV capacity and usable storage capacity. Example: with an 8 kWp PV system, the battery should not be larger than about 12.8 kWh. These are not absolute values, but rough guidelines.
Some experts also recommend oversizing the battery, because limiting the depth of discharge to around 50% can extend the service life of lithium-ion storage systems. However, this comes with additional costs, so we mention it here neutrally as part of a comprehensive overview.
In the future, electricity may also be charged into the battery from the public grid at low prices and discharged later when prices are higher. This can be an incentive to choose a larger battery in order to generate additional savings or revenue.
Most systems today use lithium-ion batteries. In off-grid applications, lead-acid batteries are still sometimes used, but since this is a separate segment and lead-acid has also become rare here, we will not go into that type in more detail.
Among lithium-ion batteries, lithium iron phosphate (LFP) is considered the safest and is generally more environmentally friendly than other lithium-ion chemistries. Key advantages include long service life, a high usable depth of discharge, and excellent safety. A lifespan of around 20 years is often stated for these storage systems. The depth of discharge is typically between 80% and 98% of the total capacity.
The function of supplying electricity during a grid outage using a battery is most commonly referred to as a backup power function or emergency power function, although additional terms have become more common in recent years. In principle, there are two main options.
Most inverters provide a second output for the loads that should continue running during a power outage. In many cases, the switchover is so fast that manufacturers even advertise an uninterruptible power supply (UPS) function—i.e., in the millisecond range. With this solution, selected circuits in the main distribution board are connected to this backup output. The loads on these circuits are then supplied during a grid outage.
The second option is to disconnect the system from the public grid (grid isolation) in order to supply the entire house with electricity. In most cases, this grid disconnection is carried out automatically using a dedicated switching / isolation device.
Note: In both cases, you are limited by the maximum output power of the system. This determines how many devices can run at the same time.
This refers to how many phases your PV and battery storage system should operate on. In most cases, three phases are supplied by the utility to the building, and all loads are powered across these phases. With three-phase inverters, connection and operation are usually straightforward.
With single-phase systems, people often have concerns because the PV and storage system is connected to only one phase, while the other two phases are not directly supplied by it. In practice, this is handled via the electricity meter: the meter is netting/summing across phases, meaning it offsets the power supplied by the battery against the electricity drawn from the utility.
The main limitation is phase imbalance: a single-phase system is restricted by the allowable imbalance between the three phases—typically no more than 4.6 kVA difference. Feel free to contact us if you have any questions.
Advantages of AC systems
If a battery storage system is to be retrofitted into an existing PV system, this is usually easiest with an AC-coupled system. These can be adapted flexibly because they operate independently from the PV system.
With some devices, a load-dependent emergency generator can also be integrated into the system during a grid outage. However, an AC storage system with backup capability can only discharge the battery during an outage. Once the battery is empty, no further energy can be supplied.
Note: From an energy-efficiency perspective, this setup typically has the lowest overall efficiency and can mean losses of up to 30%.
Advantages of DC systems
With DC-coupled systems, the PV inverter and the battery inverter are combined in one device. This makes installation easier and requires less space. Another advantage is lower conversion losses, because the battery can be charged directly with PV DC power. In addition, many of these systems can recharge from solar during a grid outage—meaning if the sun is shining, the battery will be charged again even without the grid.
Here you can find several storage bundles that include a battery and compatible battery inverters. Some bundles also include part of the required accessories. If you’re not sure which storage system is right for you, we’ll be happy to advise you personally. Simply contact us and we’ll prepare an offer for the components you need.
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Consumer information on batteries and rechargeable batteries used in energy storage systems: By law, consumers are obliged to return used batteries and solar storage batteries. This obligation applies regardless of battery type and regardless of the manufacturer or seller. Batteries must not be disposed of in household waste. We are obliged to accept used batteries free of charge. Consumers can return/send batteries after use to our shipping warehouse: TST Solarstrom OHG Baron-Riederer-Str. 48 84337 Schönau Germany However, we are unable to reimburse return shipping costs. As an end user, you can also usually return batteries in household quantities to collection points set up by public waste disposal authorities (e.g. municipal recycling centers). Batteries or rechargeable batteries that contain hazardous substances are marked with the symbol of a crossed-out wheeled bin. The crossed-out wheeled bin indicates that the battery must not be disposed of in household waste. Near the bin symbol you will find the chemical symbol of the hazardous substance, with the following meaning: "Cd": the battery contains more than 0.002% cadmium by mass; "Pb": the battery contains more than 0.004% lead by mass; "Hg": the battery contains more than 0.0005% mercury by mass. |
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